Foraging activity of roving herbivorous reef fish (Acanthuridae and Scaridae) in eastern Brazil: influence of resource availability and interference competition

Foraging activity of roving herbivorous reef fish (RHs) from families Acanthuridae (surgeonfish; three species) and Scaridae (parrotfish; five species), and its relationship to resource availability and interference competition, was studied in the largest South Atlantic Reef complex (Abrolhos Bank, eastern Brazil). Observations were undertaken at four sites differing in resource availability and competitors' abundance (i.e. RHs and territorial herbivores from genus Stegastes). Turf algae (TA) were selected by most species in most sites, while other food items were generally avoided. Surgeonfish had higher feeding rates than parrotfish, the former grazing more frequently on fleshy algae (FA) and the latter on crustose calcareous algae (CCA). Both surgeonfish and parrotfish interacted agonistically most frequently with damselfish, followed by confamilial interactions. Despite these consistent patterns, feeding rates, food selection and frequency of agonistic interactions differed significantly between sites for most species. Bite rates on CCA and FA were disproportionally higher in sites where such items were more available, leading to significant spatial variation in grazing selectivity (i.e. positive rather than negative selection of CCA and 'lower avoidance' of FA). Although agonistic interactions were more frequent at sites where herbivorous fish (both roving and territorial) were most abundant, there was no clear relationship between interference competition and foraging patterns. These results indicate that the scarcity of other food may induce RHs to consume the dominant resources. They also support the hypotheses that RHs are unable to clear large tracts of reef surface of FA once these have proliferated, and that territorial herbivores do not limit the access of RHs to particular resources.